This invention relates generally to rotary transformer systems and, more particularly, to an improved shunt calibration system for a rotary transformer transducer system.
Rotating shaft torque transducers often utilize the maintenance advantages of rotary transformers for signal transfer. Rotating transformers differ from conventional transformers only in that either the primary or secondary winding is rotating. Such rotary transformers are generally described in U.S. Pat. No. 3,611,230 entitled Rotary Transformer Structure. One transformer is used only to transmit the AC bridge supply voltage to the strain gage bridge, and a second transformer is used to pick off the output voltage. Thus, there is no direct contact between the rotating and stationary elements of the sensor. Due to this isolation, an external or dummy bridge or a Ground Reference Network, as disclosed in U.S. Pat. No. 3,790,811 must be used on the instrument side of the transformers to provide a current return path for the AC carrier instruments used.
When a rotary transformer system is utilized in conjunction with a strain gage bridge transducer to measure the torque on the shaft, it is desirable to calibrate the strain gage bridge relative to the electrical instrumentation. Such calibration is necessary because of normal variations in the electrical equipment which provides the multiple functions of supplying the signal to the strain gage bridge and which receives the output signal from the strain gage bridge. In addition, it is sometimes desirable to calibrate the equipment while the shaft is rotating.
To calibrate a strain gage bridge in general, a well known technique is to apply a resistor across one leg of the bridge. This technique is called shunt calibration because the resistor is connected in shunt or parallel across the bridge.
Prior to the present invention, calibration of the transducer in a rotary transformer system was very complex. The dummy bridge permitted a convenient method of performing a shunt calibration that is otherwise not possible on the shaft. This method of shunt calibration which is external to or removed from the rotating transducer is, in general, valid; however, it has certain limitations which restrict its use to controlled conditions which are difficult to obtain in actual practice.
Input impedance unbalance, found in some instruments, necessitates the use of a resistor-capacitor correction network with the dummy bridge. With an external shunt calibration system, the simulated shunt phase and the actual signal phase are frequency dependent and usually will not match. The shunt signal must be shifted to be in phase with the actual signal at the carrier drive frequency. This is accomplished by adjusting the values of the R-C network. The frequency sensitivity of this system limits instrument to instrument compatibility.
Hence, it is the object of this invention to eliminate the problem of instrument sensitivity by presenting the shunt calibration directly on the rotating strain gage bridge. The solution was to provide a means of switching-in an appropriate shunt resistor without physical contact between the static and rotating members and thus maintaining the advantages of a rotary transformer signal transfer system.
The system described herein makes use of a magnetically actuated switch, mounted on the rotating shaft, that is activated by introducing a magnetic field of sufficient strength. The switch, which is placed in series with a calibration resistor parallel to an appropriate leg of the bridge, can be activated and used in static and dynamic conditions regardless of shaft position by utilizing magnetically conductive rings.